This invention relates in general to spectrum analyzers and in particular, to a spectrum analyzer and encoder employing spatial modulation of radiation dispersed by wavelength.
Radiation spectral analysis is presently carried out in a number of ways. Dispersive and Fourier transform based analyzers are for high resolution and can be used for many different applications so that they are more versatile than existing application-specific instruments and procedures. While these analyzers offer superior spectral performance, they tend to be expensive, slow and are not portable. For most applications, these instruments offer a resolution which is largely unnecessary. Many applications require measurements only at several wavelengths so that most of the data taken over the entire complete spectrum using these instruments is discarded and not used at all for such applications. Such analyzers may also be too large and heavy for many practical applications.
In contrast, a non-dispersive approach to spectral analysis employs interference filters of fixed frequency passbands to perform given analytical functions. To perform the measurement, the light signal containing a number of wavelength components is propagated through one or more interference filters which are characterized by a center wavelength and bandwidth. The non-dispersive approach is advantageous over the Fourier transform and dispersive spectrum analyzers in that the non-dispersive approach is cheaper and measures the minimum amount of spectral data required to perform a given analytical function. However, if the analytical function requires a significant number of filters, the system's signal-to-noise ratio is reduced as the total energy measured in a given filter over time is inversely related to the number of filters. Furthermore, if a spectrum analyzer using this approach is configured for a first application, the filters used in the device may have to be replaced, or the number of filters changed, in order to adapt the analyzer to a second application. Therefore, even though the non-dispersive approach may be cheaper and does not measure-unnecessary data as compared to the dispersive and Fourier transform approaches, the present non-dispersive approach has its limitations.
Another type of optical spectrum analyzer, which is best described as a hybrid between dispersive and non-dispersive instruments, is the Hadamard spectrometer. The Hadamard spectrometer includes a spatial light modulator, comprised of a disc made of an opaque material with slots therein that reflect or transmit light, where the slots have uniform transmittance or reflectance, a technique which is generally referred to as a binary amplitude modulation or chopping. A light beam is dispersed according to wavelength onto the disc so that different wavelength components may be reflected by or transmitted through different slots. The disc is rotated about a rotation axis and the slots are selectively spaced at different radii from the axis to form a number of different channels for detecting corresponding wavelength components of the beam. The light reflected or transmitted by all of the channels are directed to the same detector. In order to differentiate the intensity of the wavelength component transmitted or reflected by a slot for one channel from that of another wavelength component for another channel, the disc is sequentially stepped through an application specific number of steps, each step defining a binary pattern, which defines one equation in a system of simultaneous equations for the amplitudes of the wavelength components. This set of simultaneous equations is then solved to yield the intensity for each channel prior to any specific analytical function, an approach which is cumbersome and time consuming.
None of the above approaches is entirely satisfactory. It is, therefore, desirable to provide an improved spectrum analyzer where the above-noted disadvantages are avoided or significantly diminished, in particular, where the encoding and demodulation are both generalized and significantly simplified such that the details of the spectrum analyzer can be rendered to a single application specific hardware component.